1. Field of the Invention
The present invention relates to a semiconductor device, a display device, a light-emitting device, a liquid crystal display device, and a memory device; driving methods thereof; and manufacturing methods thereof. In particular, the present invention relates to a flip flop or a shift register. Further, the present invention relates to a display device having a flip flop or a shift register.
2. Description of the Related Art
A shift register which sequentially outputs signals (e.g., sampling signals) by input of a clock signal and a start signal is used for various circuits. For example, in a display device including a plurality of pixels arranged in matrix, a shift register is used in a source signal line driver circuit or a gate signal line driver circuit which selects a pixel or inputs a signal to the selected pixel. A shift register can be used for a driver circuit for memories arranged in matrix.
In recent years, with the increase of large display devices such as liquid crystal televisions, display devices such as liquid crystal display devices and light-emitting devices have been actively developed. In particular, a technique for forming a pixel circuit and a driver circuit in a display device over the same insulating substrate by using thin film transistors or the like including semiconductor layers provided over the insulating substrate has been actively developed, because the technique greatly contributes to reduction in power consumption and cost, for example. The driver circuit formed over the insulating substrate is connected to a circuit including a controller IC through an FPC, for example, and its operation is controlled by the controller IC.
Examples of the driver circuit in the display device are a source signal line driver circuit and a gate signal line driver circuit. The examples of the driver circuit have shift registers which are one kind of electronic circuits. The shift register includes a plurality of sequential circuits such as flip-flops.
A shift register using a flip flop generally uses a clock signal and an inverted clock signal. However, whether in the case where an inverted clock signal is input from the outside of a driver circuit or an inverted clock signal is generated in the driver circuit, power consumption, the number of wirings, or the number of transistors is increased; accordingly, a flip flop or a shift register which operates only with a clock signal without an inverted clock signal has been used in some cases. As examples of such a shift register, Patent Document 1 and Patent Document 2 can be given.